Chemical gradients in the Milky Way from the RAVE data

I. Dwarf stars

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
e-mail: corrado@ari.uni-heidelberg.de
² Leibniz Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ Observatoire de Strasbourg, Université de Strasbourg, CNRS 11 rue de l’Université, 67000 Strasbourg, France
⁴ Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, PR1 2HE, UK
⁷ Monash Centre for Astrophysics, School of Mathematical Sciences, Monash University, Clayton, VIC, 3800, Australia
⁸ INAF Osservatorio Astronomico di Padova, via dell’Osservatorio 8, 36012 Asiago, Italy
⁹ Department of Physics and Astronomy, Padova University, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
¹⁰ University of Victoria, PO Box 3055, Station CSC, Victoria, BC V8W 3P6, Canada
¹¹ Department of Physics & Astronomy, Macquarie University, NSW 2109 Sydney, Australia
¹² Research Centre for Astronomy, Astrophysics and Astrophotonics, Macquarie University, NSW 2109 Sydney, Australia
¹³ Australian Astronomical Observatory, PO Box 915, NSW 1670 North Ryde, Australia
¹⁴ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, UK
¹⁵ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
¹⁶ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
¹⁷ Center of Excellence SPACE-SI, Askerceva cesta 12, 1000 Ljubljana, Slovenia

Received: 14 June 2013
Accepted: 27 August 2013

Abstract

Aims. We aim at measuring the chemical gradients of the elements Mg, Al, Si, and Fe along the Galactic radius to provide new constraints on the chemical evolution models of the Galaxy and Galaxy models such as the Besançon model. Thanks to the large number of stars of our RAVE sample we can study how the gradients vary as function of the distance from the Galactic plane.

Methods. We analysed three different samples selected from three independent datasets: a sample of 19 962 dwarf stars selected from the RAVE database, a sample of 10 616 dwarf stars selected from the Geneva-Copenhagen Survey (GCS) dataset, and a mock sample (equivalent to the RAVE sample) created by using the GALAXIA code, which is based on the Besançon model. The three samples were analysed by using the very same method for comparison purposes. We integrated the Galactic orbits and obtained the guiding radii (R_g) and the maximum distances from the Galactic plane reached by the stars along their orbits (Z_max). We measured the chemical gradients as functions of R_g at different Z_max.

Results. We found that the chemical gradients of the RAVE and GCS samples are negative and show consistent trends, although they are not equal: at Z_max< 0.4 kpc and 4.5 <R_g(kpc) < 9.5, the iron gradient for the RAVE sample is d [Fe/H] /dR_g = −0.065 dex kpc^-1, whereas for the GCS sample it is d [Fe/H] /dR_g = −0.043 dex kpc^-1 with internal errors of ±0.002 and ±0.004 dex kpc^-1, respectively. The gradients of the RAVE and GCS samples become flatter at larger Z_max. Conversely, the mock sample has a positive iron gradient of d [Fe/H] /dR_g = +0.053 ± 0.003 dex kpc^-1 at Z_max< 0.4 kpc and remains positive at any Z_max. These positive and unrealistic values originate from the lack of correlation between metallicity and tangential velocity in the Besançon model. In addition, the low metallicity and asymmetric drift of the thick disc causes a shift of the stars towards lower R_g and metallicity which, together with the thin-disc stars with a higher metallicity and R_g, generates a fictitious positive gradient of the full sample. The flatter gradient at larger Z_max found in the RAVE and the GCS samples may therefore be due to the superposition of thin- and thick-disc stars, which mimicks a flatter or positive gradient. This does not exclude the possibility that the thick disc has no chemical gradient. The discrepancies between the observational samples and the mock sample can be reduced by i) decreasing the density; ii) decreasing the vertical velocity; and iii) increasing the metallicity of the thick disc in the Besançon model.